Ceramic honeycomb structure and its production method

ABSTRACT

A ceramic honeycomb structure comprising large numbers of cells partitioned by porous cell walls, the cell walls having (a) porosity of 50-80%, and when measured by mercury porosimetry, (b) a median pore diameter being 25-50 μm, (c) (i) a cumulative pore volume in a pore diameter range of 20 μm or less being 25% or less of the total pore volume, (ii) a cumulative pore volume in a pore diameter range of more than 20 μm and 50 μm or less being 50% or more of the total pore volume, and (iii) a cumulative pore volume in a pore diameter range of more than 50 μm being 12% or more of the total pore volume.

FIELD OF THE INVENTION

The present invention relates to a ceramic honeycomb structure used forcleaning exhaust gases discharged from internal engines of automobiles,etc., and its production method.

BACKGROUND OF THE INVENTION

To remove harmful substance such as particulate matter (PM) and NOx(nitrogen oxide), HC (hydrocarbon), CO (carbon monoxide), SOx (sulfuroxide), etc. from exhaust gases discharged from filters, internalengines of construction machines, industrial machines, etc., ceramichoneycomb structures are used as carriers for catalysts for cleaningexhaust gases, and filters for capturing fine particles. As shown inFIGS. 1( a) and 1(b), a ceramic honeycomb structure 1 comprises largenumbers of cells 13 partitioned by porous cell walls 12 and extending inan exhaust-gas-flowing direction. A ceramic honeycomb structure used asa catalyst carrier carries an exhaust-gas-cleaning catalyst on its cellwall surfaces and in pores inside the cell walls, so that an exhaust gaspassing through the ceramic honeycomb structure is cleaned by thecatalyst.

When an exhaust gas is cleaned by such a ceramic honeycomb structure,harmful substance in the exhaust gas should come into efficient contactwith a catalyst carried by cell walls to improve cleaning efficiency.Generally conducted to this end is to increase a cell density bydecreasing the opening areas of cells, thereby obtaining a large contactarea with a catalyst. However, increase in the cell density results inincrease in pressure loss.

To solve such problem, JP 2006-517863 A discloses a catalyst supportcomprising a porous ceramic honeycomb structure having pluralities ofparallel cells penetrating from the inlet ends to the outlet ends, whichhas porosity of more than 45% by volume, as well as a network structurehaving communicating pores having a narrow pore diameter distributionhaving a median pore diameter of more than 5 μm and less than 30 μm. JP2006-517863 A describes that this catalyst support (ceramic honeycombstructure) can bear a higher percentage of a catalyst without sufferingpressure decrease.

WO 2007/026803 A1 discloses a ceramic honeycomb structure comprisingpluralities of cells penetrating between two end surfaces, which areconstituted by porous cell walls having large numbers of pores, andplugs for sealing the cells at either end surfaces or inside the cells,the cell walls having permeability of 7×10⁻¹² m² to 4×10⁻⁸ m². WO2007/026803 A1 describes that this ceramic honeycomb structure exhibitsexcellent cleaning efficiency with small pressure loss.

In the ceramic honeycomb structures described in JP 2006-517863 A and WO2007/026803 A1, however, pores in the cell walls may be clogged by acatalyst applied to the cell wall surfaces and pores in the cell walls,resulting in large pressure loss in a catalyst-carrying ceramichoneycomb structure. As a result, a catalyst carried in pores in thecell walls is unlikely used effectively, failing to obtain improvedcleaning efficiency. Particularly in a ceramic honeycomb structure(catalyst carrier) having no plugs, like the catalyst support describedin JP 2006-517863 A, an exhaust gas may not be easily flowable throughpores in the cell walls, resulting in poor cleaning efficiency. Further,the ceramic honeycomb structure of WO 2007/026803 A1 has poor strength,because its cell walls per se have small strength due to largepermeability.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a ceramichoneycomb structure for cleaning an exhaust gas, which has high strengthwithout plugs, while securing good flowability of an exhaust gas throughpores in cell walls, and which exhibits high capability of removingharmful substance with small pressure loss when carrying a catalyst.

SUMMARY OF THE INVENTION

As a result of intensive research in view of the above object, theinventors have found that a ceramic honeycomb structure comprising largenumbers of cells partitioned by porous cell walls has high strength aswell as high flowability of an exhaust gas through pores, when the cellwalls has (a) porosity of 50-80%, and when measured by mercuryporosimetry, (b) a median pore diameter being 25-50 μm, (c) a cumulativepore volume in a pore diameter range of 20 μm or less being 25% or lessof the total pore volume, (d) a cumulative pore volume in a porediameter range of more than 20 μm and 50 μm or less being 50% or more ofthe total pore volume, and (e) a cumulative pore volume in a porediameter range of more than 50 μm being 12% or more of the total porevolume. The present invention has been completed based on such finding.

Thus, the ceramic honeycomb structure of the present invention compriseslarge numbers of cells partitioned by porous cell walls, the cell wallshaving

-   (a) porosity of 50-80%; and-   when measured by mercury porosimetry,-   (b) a median pore diameter being 25-50 μm;-   (c) (i) a cumulative pore volume in a pore diameter range of 20 μm    or less being 25% or less of the total pore volume;-   (ii) a cumulative pore volume in a pore diameter range of more than    20 μm and 50 μm or less being 50% or more of the total pore volume;    and-   (iii) a cumulative pore volume in a pore diameter range of more than    50 μm being 12% or more of the total pore volume.

The opening area ratio of pores open on the cell wall surfaces (thetotal opening area of pores per a unit cell wall surface area) ispreferably 30% or more.

The median opening diameter of pores open on the cell wall surfaces ispreferably 60 μm or more (expressed by equivalent circle diameter).

With respect to pores open on the cell wall surfaces, a cumulativeopening area in a pore opening diameter range of 30 μm or less(expressed by equivalent circle diameter) is preferably 20% or less ofthe total opening area.

With respect to pores open on the cell wall surfaces, a cumulativeopening area in a pore opening diameter range of 100 μm or more(expressed by equivalent circle diameter) is preferably 30-70% of thetotal opening area.

The cell walls preferably have permeability of 10×10⁻¹² m² to 30×10⁻¹²m².

The ceramic honeycomb structure preferably has an A-axis compressionstrength of 1.0 MPa or more.

The method of the present invention for producing a ceramic honeycombstructure comprises the steps of blending material powder comprising acordierite-forming material and a pore-forming material to obtain amoldable material, extruding the moldable material to obtain ahoneycomb-shaped green body, and drying and sintering the green body toobtain a ceramic honeycomb structure,

-   (a) the cordierite-forming material comprising 10-25% by mass of    silica having a median particle diameter of 10-60 μm;-   (b) the pore-forming material having-   (i) a median particle diameter of more than 70 μm and 200 μm or    less,-   (ii) in a curve of a cumulative volume (cumulative volume of    particles up to a particular particle diameter) to a particle    diameter, a particle diameter D90 at a cumulative volume    corresponding to 90% of the total volume being 90-250 μm, and a    particle diameter D10 at a cumulative volume corresponding to 10% of    the total volume being 15-160 μm, and-   (iii) a particle diameter distribution deviation SD being 0.3 or    less; and-   (c) the amount of the pore-forming material added being 1-20% by    mass per the cordierite-forming material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a front view schematically showing an example of theceramic honeycomb structures of the present invention.

FIG. 1( b) is a partial cross-sectional view schematically showing anexample of the ceramic honeycomb structures of the present invention.

FIG. 2 is an electron photomicrograph showing a cell wall surface of theceramic honeycomb structure of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of present invention will be specifically explainedbelow without intention of restricting the present invention thereto.Proper modifications, improvements, etc. may be made based on the commonknowledge of those skilled in the art, within the scope of the presentinvention.

The ceramic honeycomb structure of the present invention comprises largenumbers of cells partitioned by porous cell walls, the cell walls having

-   (a) porosity of 50-80%; and-   when measured by mercury porosimetry,-   (b) a median pore diameter being 25-50 μm;-   (c) (i) a cumulative pore volume in a pore diameter range of 20 μm    or less being 25% or less of the total pore volume,-   (ii) a cumulative pore volume in a pore diameter range of more than    20 μm and 50 μm or less being 50% or more of the total pore volume,    and-   (iii) a cumulative pore volume in a pore diameter range of more than    50 μm being 12% or more of the total pore volume.

With a catalyst carried on the ceramic honeycomb structure of thepresent invention, an exhaust gas can easily flow through pores insidethe cell walls, resulting in an exhaust-gas-cleaning ceramic honeycombstructure having high capability of catalytically removing harmfulsubstance with small pressure loss.

(a) Porosity of Cell Walls

The cell walls have porosity of 50-80%. When the cell walls haveporosity of less than 50%, a catalyst-carrying ceramic honeycombstructure cannot keep low pressure loss. When the porosity exceeds 80%,the ceramic honeycomb structure does not have practically acceptablestrength. The porosity is preferably 55-75%, more preferably 58-72%.

(b) Median Pore Diameter of Cell Walls

The median pore diameter measured by mercury porosimetry is 25-50 μm. Acatalyst-carrying ceramic honeycomb structure may suffer poor pressureloss characteristics at a median pore diameter of less than 25 μm, andmay practically suffer low strength at a median pore diameter of morethan 50 μm. The median pore diameter is preferably 28-45 μm, morepreferably 30-40 μm.

(c) Pore Diameter Distribution of Cell Walls

When the cell walls are measured by mercury porosimetry, (i) acumulative pore volume in a pore diameter range of 20 μm or less is 25%or less of the total pore volume, (ii) a cumulative pore volume in apore diameter range of more than 20 μm and 50 μm or less is 50% or moreof the total pore volume, and (iii) a cumulative pore volume in a porediameter range of more than 50 μm is 12% or more of the total porevolume. With the cell walls having such pore diameter distribution, acatalyst-carrying ceramic honeycomb structure permits an exhaust gas toeasily flow through pores in the cell walls while keeping high strengthas a ceramic honeycomb structure, thereby exhibiting high capability ofcatalytically removing harmful substance with small pressure loss.

When the cumulative pore volume in a pore diameter range of 20 μm orless is more than 25% of the total pore volume, a smaller percentage ofpores have diameters of more than 20 μm, resulting in more pressureloss. The cumulative pore volume in a pore diameter range of 20 μm orless is preferably 3-22%, more preferably 5-18%.

When the cumulative pore volume in a pore diameter range of more than 20μm and 50 μm or less is less than 50% of the total pore volume, acatalyst-carrying ceramic honeycomb structure has larger percentages offine pores and coarse pores, resulting in poor pressure losscharacteristics and low strength. The cumulative pore volume in a porediameter range of more than 20 μm and 50 μm or less is preferably55-80%, more preferably 60-75%.

When the cumulative pore volume in a pore diameter range of more than 50μm is less than 12% of the total pore volume, a catalyst-carryingceramic honeycomb structure has poor pressure loss characteristics. Thecumulative in a pore diameter range of more than 50 μm is preferably15-30%, more preferably 18-25%.

The pore distribution deviation σ is preferably 0.35 or less. When thepore distribution deviation σ is more than 0.35, there is a largerpercentage of pores deteriorating pressure loss and strength. The poredistribution deviation σ is preferably 0.32 or less, more preferably0.30 or less. The pore distribution deviation σ meets the relation ofσ=log(d₂₀)−log(d₈₀), wherein d₂₀ represents a pore diameter (μm) at acumulative pore volume corresponding to 20% of the total pore volume,and d₈₀ represents a pore diameter (μm) at a cumulative pore volumecorresponding to 80% of the total pore volume, d₈₀<d₂₀, in a curve of acumulative pore volume (cumulative volume of pores in a range from themaximum pore diameter to a particular pore diameter) to a pore diameter.The relation between a pore diameter and a cumulative pore volume can bedetermined by mercury porosimetry. The measurement is preferablyconducted by a mercury porosimeter.

(d) Opening Area Ratio of Pores Open on Cell Wall Surfaces

The opening area ratio of pores open on the cell wall surfaces (totalopening area of pores per a unit cell wall surface area) is preferably30% or more. When the opening area ratio is less than 30%, an exhaustgas cannot flow through pores in the cell walls easily, likely resultingin less contact with a catalyst carried on the cell wall surfaces orinside their pores, and thus lower cleaning efficiency.

As shown in FIG. 2, for example, the opening area ratio of pores open onthe cell wall surfaces is determined by measuring the total opening areaof pores on an electron photomicrograph of a cell wall surface with afield area 2 times or more the cell wall thickness (in a field of 600μm×600 μm or more, for example, when the cell wall thickness is 300 μm)by an image analyzer (for example, Image-Pro Plus ver.6.3 available fromMedia Cybernetics), and dividing it by the field area measured.

(e) Median Opening Diameter of Pores Open on Cell Wall Surfaces

The median opening diameter of pores open on the cell wall surfaces ispreferably 60 μm or more, when expressed by an equivalent circlediameter. When the median opening diameter is less than 60 μm, acatalyst-carrying ceramic honeycomb structure may have large pressureloss.

The median opening diameter of pores open on the cell wall surfaces isan equivalent circle diameter of pores at a cumulative areacorresponding to 50% of the total pore area, in a graph plotting acumulative opening area of pores on the cell wall surfaces [a cumulativeopening area of pores in a range of a particular equivalent circlediameter (diameter of a circle having the same area as the opening areaof each pore) or less] to the equivalent circle diameters of openingpores. The opening areas and equivalent circle diameters of pores can bedetermined by analyzing an electron photomicrograph of the cell wallsurface by an image analyzer (for example, Image-Pro Plus ver.6.3available from Media Cybernetics).

(f) Pore Diameter Distribution on Cell Wall Surfaces

(i) Cumulative Opening Area of Pores Having Opening Diameters of 30 μmor Less on Cell Wall Surfaces

The cumulative opening area of pores having opening diameters (eachexpressed by an equivalent circle diameter) of 30 μm or less on the cellwall surfaces is preferably 20% or less of the total opening area. Whenthe cumulative opening area in a pore opening diameter range of 30 μm orless is more than 20% of the total opening area, a catalyst-carryingceramic honeycomb structure may have large pressure loss. The cumulativeopening area in a pore opening diameter range of 30 μm or less ispreferably 17% or less, more preferably 15% or less, of the totalopening area.

(ii) Cumulative Opening Area of Pores Having Opening Diameters of 100 μmor More on Cell Wall Surfaces

The cumulative opening area of pores having opening diameters (eachexpressed by an equivalent circle diameter) of 100 μm or more on thecell wall surfaces is preferably 30-70% of the total opening area. Whenthe cumulative opening area in a pore opening diameter range of 100 μmor more is less than 30 of the total opening area, an exhaust gas isless flowable through pores in the cell walls, likely resulting in lesscontact with a catalyst carried on the cell wall surfaces or insidetheir pores, and thus lower cleaning efficiency. When it is more than70%, the ceramic honeycomb structure has low strength. The cumulativeopening area in a pore opening diameter range of 100 μm or more ispreferably 33-65%, more preferably 35-60%, of the total opening area.

(g) Permeability

The cell walls preferably have permeability of 10×10⁻¹² m² to 30×10⁻¹²m₂. With the cell walls having permeability of 10×10⁻¹²m² to 30×10⁻¹²m², an exhaust-gas-cleaning ceramic honeycomb structure having highercapability of catalytically removing harmful substance and sufferingsmaller pressure loss even after carrying a catalyst can be obtained. Acatalyst-carrying ceramic honeycomb structure has large pressure loss ata cell wall permeability of less than 10×10⁻¹² m², while it may have lowcell wall strength at a cell wall permeability of more than 30×10⁻¹² m².The permeability of the cell walls is preferably 12×10⁻¹² m² to 28×10⁻¹²m².

(h) A-Axis Compression Strength

The ceramic honeycomb structure preferably has an A-axis compressionstrength of 1.0 MPa or more. With an A-axis compression strength of 1.0MPa or more, an exhaust-gas-cleaning ceramic honeycomb structure canhave higher capability of catalytically removing harmful substance withsmaller pressure loss even after carrying a catalyst. When the A-axiscompression strength is less than 1.0 MPa, the ceramic honeycombstructure may not be able to keep practically acceptable strength. theA-axis compression strength is preferably 1.2 MPa or more.

(i) Thermal Expansion Coefficient

The ceramic honeycomb structure preferably has a thermal expansioncoefficient of 15×10⁻⁷/° C. or less between 20° C. and 800° C. Theceramic honeycomb structure having such a thermal expansion coefficientis sufficiently practically acceptable as a catalyst carrier forcleaning an exhaust gas discharged from internal engines, or a filterfor capturing fine particles therein, because of high heat shockresistance. The thermal expansion coefficient is preferably 3×10⁻⁷ to13×10⁻⁷.

(j) Other Structural Features

The ceramic honeycomb structure preferably has an average cell wallthickness of 0.10-0.50 mm and an average cell density of 150-500 cpsi.With such average cell wall thickness and average cell density, theceramic honeycomb structure can be turned to an exhaust-gas-cleaningceramic honeycomb structure having small pressure loss while keepinghigh strength. The cell walls have low strength at an average cell wallthickness of less than 0.10 mm, and have difficulty in keeping lowpressure loss at an average cell wall thickness of more than 0.50 mm.The average cell wall thickness is preferably 0.15-0.45 mm. The cellwalls have low strength at an average cell density of less than 150 cpsi(23.3 cells/cm²), and have difficulty in keeping low pressure loss at anaverage cell density of more than 500 cpsi (77.5 cells/cm²). The averagecell density is preferably 170-300 cpsi, more preferably 160-280 cpsi.

Materials for the ceramic honeycomb structure are preferablyheat-resistant ceramics comprising as main crystals, alumina, mullite,cordierite, silicon carbide, silicon nitride, zirconia, aluminumtitanate, lithium aluminum silicate, etc., because the ceramic honeycombstructure is used as a catalyst carrier for cleaning exhaust gases or afilter for capturing fine particles therein. Among them, ceramicscomprising cordierite as main crystals for excellent heat shockresistance with low thermal expansion are preferable. When the maincrystal phase is cordierite, other crystal phases such as spinel,mullite, sapphirine, etc. may be contained, and glass components mayalso be contained.

[2] Production Method of Ceramic Honeycomb Structure

The method of the present invention for producing a ceramic honeycombstructure comprises the steps of blending a material powder comprising acordierite-forming material and a pore-forming material to obtain amoldable material, extruding the moldable material to obtain ahoneycomb-shaped green body, and drying and sintering the green body toobtain a ceramic honeycomb structure,

-   (a) the cordierite-forming material comprising 10-25% by mass of    silica having a median particle diameter of 10-60 μm;-   (b) the pore-forming material having-   (i) a median particle diameter of more than 70 μm and 200 μm or    less,-   (ii) in a curve of a cumulative volume (cumulative volume of    particles up to a particular particle diameter) to a particle    diameter, a particle diameter D90 at a cumulative volume    corresponding to 90% of the total volume being 90-250 μm, and a    particle diameter D10 at a cumulative volume corresponding to 10% of    the total volume being 15-160 μm, and-   (iii) a particle diameter distribution deviation SD being 0.3 or    less, and-   (c) the amount of the pore-forming material added being 1-20% by    mass per the cordierite-forming material.

Such method can produce a ceramic honeycomb structure comprising cellwalls having porosity of 50-80%, and when measured by mercuryporosimetry, a median pore diameter being 25-50 μm, (i) a cumulativepore volume in a pore diameter range of 20 μm or less being 25% or lessof the total pore volume, (ii) a cumulative pore volume in a porediameter range of more than 20 μm and 50 μm or less being 50% or more ofthe total pore volume, and (iii) a cumulative pore volume in a porediameter range of more than 50 μm being 12% or more of the total porevolume.

(a) Cordierite-Forming Material

The cordierite-forming material is obtained by mixing a silica sourcepowder, an alumina source powder and a magnesia source powder, to formcordierite as a main crystal having a chemical composition comprising42-56% by mass of SiO₂, 30-45% by mass of Al₂O₃, and 12-16% by mass ofMgO. Pores in the ceramics comprising cordierite as a main crystal havepores formed by sintering silica, and pores generated by burning thepore-forming material.

10-25% of silica having a median particle diameter of 10-60 μm is usedfor the cordierite-forming material. When silica has a median particlediameter of less than 10 μm, there are many fine pores deterioratingpressure loss characteristics. When it exceeds 60 μm, there are manycoarse pores lowering strength. The median particle diameter of silicais preferably 35-55 μm.

(b) Pore-Forming Material

The pore-forming material has a median particle diameter of more than 70μm and 200 μm or less. When the median particle diameter of thepore-forming material is 70 μm or less, relatively small pores may beformed on and in the cell walls, resulting in less effective contactwith a catalyst, and thus a lower cleaning efficiency. When the medianparticle diameter is more than 200 μm, large pores are formed, resultingin lower strength. The median particle diameter of the pore-formingmaterial is preferably 75-180 μm, more preferably 80-150 μm.

In a curve of a cumulative volume (cumulative volume of particles up toa particular particle diameter) to a particle diameter of thepore-forming material, a particle diameter D90 at a cumulative volumecorresponding to 90% of the total volume is 90-250 μm, a particlediameter D10 at a cumulative volume corresponding to 10% of the totalvolume is 15-160 μm, and a particle diameter distribution deviation SDis 0.3 or less.

When the pore-forming material has such a particle diameterdistribution, cell walls having the above pore structure are likelyobtained. The particle diameter D10 and the particle diameter D90 meetthe relation of D90>D10.

The particle diameter distribution deviation SD is a value expressed bySD=log(D80)−log(D20), wherein D20 represents a particle diameter (μm) ata cumulative volume of 20%, and D80 represents a particle diameter (μm)at a cumulative volume of 80%, in a curve of a cumulative volume [volume(%) of particles up to a particular particle diameter relative to thetotal volume] to a particle diameter, which is called a cumulativeparticle diameter distribution curve. D20<D80. The particle diameterdistribution deviation SD is preferably 0.27 or less, more preferably0.25 or less. The particle diameter (particle distribution) of thepore-forming material can be measured by a particle diameterdistribution meter (Microtrack MT3000 available from Nikkiso Co., Ltd.).

The pore-forming material may be flour, graphite, starch, solid orhollow resins (polymethylmethacrylate, polybutylmethacrylate,polyacrylates, polystyrenes, polyethylene, polyethylene terephthalate,methylmethacrylate/acrylonitrile copolymers, etc.), etc. Among them,hollow resin particles are preferable, and hollow particles ofmethylmethacrylate/acrylonitrile copolymers are more preferable. Thehollow resin particles preferably have shells as thick as 0.1-2 μm, andcontain a hydrocarbon gas, etc. They preferably contain 70-95% ofmoisture. With moisture, the resin particles are well slidable, avoidingcollapsing in mixing, blending and molding.

(c) Production Steps

The ceramic honeycomb structure is produced by blending a materialpowder comprising a cordierite-forming material and a pore-formingmaterial with a binder, water, etc. to prepare a moldable material,which is extruded from a die by a known method to form ahoneycomb-shaped green body; drying the green body; machining end andperipheral surfaces, etc. of the green body, if necessary; and thensintering the green body.

Sintering is conducted by controlling heating and cooling speeds in acontinuous or batch furnace. When the ceramic material is acordierite-forming material, it is kept at 1350-1450° C. for 1-50 hoursto sufficiently form cordierite as a main crystal, and then cooled toroom temperature to obtain a ceramic honeycomb structure formed bycordierite as a main crystal phase. The ceramic honeycomb structure maycontain other crystal phases such as spinel, mullite, sapphirine, etc.,and further glass components. Particularly when a large ceramichoneycomb structure having an outer diameter of 150 mm or more and alength of 150 mm or more is produced, the temperature-elevating speed ispreferably 0.2-10° C./hr in a binder-decomposing temperature range, forexample, between 150° C. and 350° C., and 5-20° C./hr in acordierite-forming temperature range, for example, between 1150° C. and1400° C., thereby preventing cracking in the green body in the sinteringprocess. The cooling is preferably conducted at a speed of 20-40° C./hin a temperature range of 1400° C. to 1300° C.

A catalyst comprising one or more precious metals selected from Pt, Rhand Pd may be carried on cell walls of the resultant honeycomb ceramicstructure, to provide an exhaust-gas-cleaning catalyst filter.

The honeycomb ceramic structure may be plugged in ends of or insidedesired flow paths by a known method to form a ceramic honeycomb filter.Plugs may be formed before sintering.

(1) Production of Ceramic Honeycomb Structures (Examples 1-16 andComparative Examples 1-10)

Silica powder, kaolin powder, talc powder, alumina powder, and aluminumhydroxide powder having particle diameters shown in Table 1 were mixedwith each other in proportions shown in Table 3, thereby obtaining eachcordierite-forming material powder having a chemical compositioncomprising 50% by mass of SiO₂, 35% by mass of Al₂O₃ and 13% by mass ofMgO. This cordierite-forming material powder was mixed with apore-forming material shown in Table 2 and methyl cellulose, and thenblended with water to prepare a plastically moldable ceramic materialcomprising a cordierite-forming material.

Each moldable material was extruded from an extrusion die to form ahoneycomb-shaped green body having cell wall thickness of 13 mil (0.33mm) and a cell density of 255 cpsi (39.5 cells/cm²). After drying, aperipheral portion was removed from the dried honeycomb-shaped body,which was sintered at the highest temperature of 1410° C. for 200 hoursin a furnace, thereby obtaining a ceramic honeycomb structure formed bycordierite as a main crystal. The sintered ceramic honeycomb structurewas provided with a peripheral layer of amorphous silica and colloidalsilica, and then dried to obtain a ceramic honeycomb structure having anouter diameter of 266.7 mm and a length of 304.8 mm. In each of Examples1-16 and Comparative Examples 1-10, two ceramic honeycomb structureswere produced.

(2) Measurement of Cell Wall Structure, Permeability and Strength ofCeramic Honeycomb Structure

A sample was cut out of one of the ceramic honeycomb structures obtainedin each of Examples 1-16 and Comparative Examples 1-10, to measure apore distribution in the cell walls by mercury porosimetry, thediameters of pores open on the cell wall surfaces and its distribution,permeability, and A-axis compression strength. The results are shown inTable 4.

A test piece (10 mm×10 mm×10 mm) cut out of each ceramic honeycombfilter was set in a measurement cell of Autopore III available fromMicromeritics, and the cell was evacuated. Thereafter, mercury wasintroduced into the cell under pressure to determine by mercuryporosimetry the relation between the pressure and the volume of mercuryintruded into pores in the test piece, from which the relation between apore diameter and a cumulative pore volume (cumulative pore volumedistribution curve) was determined. The mercury-intruding pressure was0.5 psi (0.35×10⁻³ kg/mm²), and constants used for calculating the porediameter from the pressure were a contact angle of 130°, and a surfacetension of 484 dyne/cm.

Porosity, pore diameter and pore distribution are obtained from acumulative pore volume distribution curve determined by mercuryporosimetry, namely from the relation between a pore diameter and acumulative pore volume. The porosity was calculated from the measuredtotal pore volume, using 2.52 g/cm³ as the true density of cordierite.The median pore diameter and the pore diameter distribution (cumulativepore volume in a pore diameter range of 20 μm or less, cumulative porevolume in a pore diameter range of more than 20 μm and 50 μm or less,and cumulative pore volume in a pore diameter range of more than 50 μm)were determined from the cumulative pore volume distribution curve.

The diameters of pores open on the cell wall surfaces and itsdistribution were determined from an electron photomicrograph of a cellwall surface in a field of 800 μm×800 μm, by an image analyzer (forexample, Image-Pro Plus ver.6.3 available from Media Cybernetics). Theopening area ratio and median opening diameter (expressed by equivalentcircle diameter) of pores open on cell wall surfaces, the cumulativeopening area in a pore opening diameter range of 30 μm or less, and thecumulative opening area in a pore opening diameter range of 100 μm ormore were calculated.

The maximum value of permeability measured by Perm Automated Porometer(registered trademark) 6.0 available from Porous Materials, Inc. whileincreasing an air flow rate from 30 cc/sec to 400 cc/sec was used aspermeability.

The A-axis compression strength was measured according to “Test MethodOf Exhaust-Gas-Cleaning Ceramic Monolith Carrier For Automobiles,” JASOstandards M505-87 of the Society of Automotive Engineers of Japan.

The thermal expansion coefficient (CTE) between 20° C. and 800° C. wasmeasured on another test piece cut out of the honeycomb filter.

(3) Evaluation of Filter Performance

The remaining one of the ceramic honeycomb structures in each ofExamples 1-16 and Comparative Examples 1-10 was provided with a catalystas follows, to evaluate the pressure loss and catalytic effect of theresultant filter.

(a) Carrying of Catalyst

A catalyst slurry comprising active alumina, platinum (Pt) as a preciousmetal, and cerium oxide (CeO₂) as a co-catalyst was applied to cell wallsurfaces and pores in the cell walls of the ceramic honeycomb structureby a suction method, to form a catalyst coating, which was dried byheating to produce a catalyst-carrying ceramic honeycomb structure.

(b) Pressure Loss

Air was supplied at a flow rate of 10 Nm³/min to a ceramic honeycombstructure fixed to a pressure loss test stand, to measure pressuredifference between the inlet side and the outlet side as initialpressure loss. The pressure loss was evaluated relative to pressure lossin Comparative Example 1 by the following standard.

Poor: The pressure loss was 0.9 times or more,

Fair: It was more than 0.7 times and 0.9 times or less,

Good: It was more than 0.5 times and 0.7 times or less, and

Excellent: It was 0.5 times or less.

The results are shown in Table 4.

(c) Catalytic Effect

An exhaust gas containing 400 ppm of NOx at a temperature of 300° C. wassupplied to a catalyst-carrying ceramic honeycomb structure, and adiesel fuel (HC) in the same amount as that of NOx was added to measurethe NOx content in the exhaust gas at the outlet of thecatalyst-carrying ceramic honeycomb structure, thereby evaluating acatalytic cleaning effect with a relative value of a NOx-removing ratioto that of Comparative Example 1 by the following standard.

Poor: It was 0.9 times or more,

Fair: It was more than 0.7 times and 0.9 times or less,

Good: It was more than 0.5 times and 0.7 times or less, and

Excellent: It was 0.5 times or less.

The results are shown in Table 4.

TABLE 1 Median Particle Material Diameter (μm) Silica A 4.4 Silica B11.0 Silica C 20.2 Silica D 41.0 Silica E 58.3 Silica F 75.0 Kaolin 3.1Talc 10.0 Alumina 6.1 Aluminum 1.8 Oxide

TABLE 2 Median Pore-Forming Particle d90 d10 Material Type Diameter (μm)(μm) (μm) SD⁽¹⁾ A Hollow Resin 42.0 83.2 13.5 0.41 B Hollow Resin 72.1105.0 15.3 0.30 C Hollow Resin 79.3 129.0 59.0 0.24 D Hollow Resin 96.5135.8 68.4 0.22 E Hollow Resin 165.0 200.0 85.0 0.26 F Hollow Resin191.0 250.0 101.0 0.22 G Hollow Resin 240.0 295.0 110.0 0.30 H Graphite150.0 235.0 41.0 0.30 Note: ⁽¹⁾SD represents a particle diameterdistribution deviation.

TABLE 3 Silica Kaolin Talc No. Type % by mass % by mass % by massExample 1 B 20.3 6.1 42 Example 2 C 20.3 6.1 42 Example 3 D 20.3 6.1 42Example 4 E 20.2 6.2 42 Example 5 D 20.3 6.1 42 Example 6 D 20.3 6.1 42Example 7 D 20.3 6.1 42 Example 8 D 20.3 6.1 42 Example 9 D 20.3 6.1 42Example 10 D 20.3 6.1 42 Example 11 D 12.0 15.4 41 Example 12 D 24.0 2.442 Example 13 D 20.3 6.1 42 Example 14 E 20.3 6.1 42 Example 15 B 20.36.1 42 Example 16 C 20.3 6.1 42 Com. Ex. 1 A 20.3 6.1 42 Com. Ex. 2 A20.3 6.1 42 Com. Ex. 3 A 20.3 6.1 42 Com. Ex. 4 F 20.3 6.1 42 Com. Ex. 5F 20.3 6.1 42 Com. Ex. 6 F 20.3 6.1 42 Com. Ex. 7 D 20.3 6.1 42 Com. Ex.8 D 20.3 6.1 42 Com. Ex. 9 D 20.3 6.1 42 Com. Ex. 10 D 20.3 6.1 42Aluminum Alumina Oxide Pore-Forming Material No. % by mass % by massType % by mass Example 1 22.1 9.5 D 9.0 Example 2 22.0 9.6 D 9.0 Example3 22.1 9.5 D 9.0 Example 4 22.1 9.5 D 9.0 Example 5 22.1 9.5 B 9.0Example 6 22.1 9.5 C 9.0 Example 7 22.1 9.5 E 9.0 Example 8 22.1 9.5 F9.0 Example 9 22.0 9.6 D 18.0 Example 10 22.0 9.6 D 2.0 Example 11 22.09.6 D 9.0 Example 12 22.1 9.5 D 9.0 Example 13 22.1 9.5 D 6.0 Example 1422.1 9.5 H 20.0 Example 15 22.0 9.6 B 7.0 Example 16 22.1 9.5 B 7.0 Com.Ex. 1 22.1 9.5 A 9.0 Com. Ex. 2 22.1 9.5 C 9.0 Com. Ex. 3 22.1 9.5 G 9.0Com. Ex. 4 22.1 9.5 C 9.0 Com. Ex. 5 22.1 9.5 A 9.0 Com. Ex. 6 22.1 9.5G 9.0 Com. Ex. 7 22.1 9.5 A 9.0 Com. Ex. 8 22.1 9.5 G 9.0 Com. Ex. 922.1 9.5 D 0.5 Com. Ex. 10 22.1 9.5 D 25.0

TABLE 4 Pores Measured by Mercury Porosimetry Total Pore Median PoreVolume Porosity Diameter Cumulative Pore Volume (%) No. (cm³/g) (%) (μm)PD⁽¹⁾ ≦ 20 20 < PD ≦ 50 Example 1 0.605 60.4 28.0 23.1 63.6 Example 20.630 61.4 30.0 20.0 64.1 Example 3 0.780 66.3 33.0 21.8 61.5 Example 40.880 68.9 35.0 17.6 65.9 Example 5 0.640 61.7 29.3 20.0 67.5 Example 60.700 63.8 30.2 24.3 60.0 Example 7 0.900 69.4 36.0 19.1 62.6 Example 81.250 75.9 41.0 20.0 52.0 Example 9 1.200 75.1 45.0 13.3 57.5 Example 100.450 53.1 28.0 24.9 56.2 Example 11 0.700 63.8 29.0 20.0 65.7 Example12 0.910 69.6 31.2 23.1 56.0 Example 13 0.715 64.3 30.4 17.5 67.8Example 14 0.896 69.3 40.0 16.3 55.8 Example 15 0.580 59.4 26.8 24.162.6 Example 16 0.610 60.9 28.0 23.3 59.8 Com. Ex. 1 0.300 43.1 21.046.7 45.0 Com. Ex. 2 0.350 46.9 25.0 25.7 62.9 Com. Ex. 3 0.399 50.129.0 27.3 58.9 Com. Ex. 4 2.000 83.4 55.0 15.0 25.0 Com. Ex. 5 1.58079.9 50.0 5.1 19.0 Com. Ex. 6 2.200 84.7 60.0 11.4 34.1 Com. Ex. 7 0.69063.5 22.0 30.4 52.2 Com. Ex. 8 0.950 70.5 45.0 20.0 43.2 Com. Ex. 90.320 44.6 22.0 29.7 65.6 Com. Ex. 10 1.920 82.9 53.0 11.5 36.5 Note:⁽¹⁾PD represents pore diameters (μm). Pores Measured by MercuryPorosimetry Pores Open on Cell Wall Surfaces Cumulative Pore PoreOpening Median Cumulative Volume (%) in distribution Area Ratio OpeningOpening Area No. PD⁽¹⁾ > 50 Deviation σ (%) Diameter (μm) (%) in OD⁽²⁾ ≦30 Example 1 13.2 0.30 32.5 68.8 24.1 Example 2 15.9 0.28 34.6 74.9 17.0Example 3 16.7 0.32 36.7 83.3 13.2 Example 4 16.5 0.28 40.5 90.5 10.2Example 5 12.5 0.28 33.3 61.7 20.0 Example 6 15.7 0.27 34.1 66.4 18.3Example 7 18.3 0.31 41.2 92.3 16.5 Example 8 28.0 0.26 55.2 122.0 9.1Example 9 29.2 0.22 54.1 109.0 11.4 Example 10 18.9 0.26 32.1 82.1 21.2Example 11 14.3 0.32 34.8 82.9 18.8 Example 12 20.9 0.32 38.8 85.1 17.9Example 13 14.7 0.32 35.1 82.2 16.7 Example 14 27.9 0.37 52.0 111.0 15.0Example 15 13.3 0.30 27.9 58.7 26.0 Example 16 17.0 0.30 32.1 59.2 23.8Com. Ex. 1 8.3 0.30 18.0 32.0 46.2 Com. Ex. 2 11.4 0.30 25.0 41.0 38.8Com. Ex. 3 13.8 0.30 32.0 58.0 30.3 Com. Ex. 4 60.0 0.38 31.0 71.0 23.4Com. Ex. 5 75.9 0.22 28.0 59.0 29.2 Com. Ex. 6 54.5 0.40 59.0 151.0 7.3Com. Ex. 7 17.4 0.36 26.8 80.2 22.2 Com. Ex. 8 36.8 0.44 41.1 93.3 11.2Com. Ex. 9 4.7 0.27 27.0 48.2 33.6 Com. Ex. 10 52.1 0.35 52.1 113.3 10.3Note: ⁽¹⁾PD represents pore diameters (μm). ⁽²⁾OD represents the openingdiameters of pores. Cumulative A-Axis Opening Area Compression CTEEvaluation of Filter (%) in 100 μm Permeability Strength (20-800° C.)Pressure Catalytic No. or more⁽¹⁾ (×10⁻¹² m²) (MPa) (×10⁻⁷/° C.) LossEffect Example 1 25.1 8.0 1.7 11.3 Fair Fair Example 2 27.9 13.0 1.610.0 Good Good Example 3 35.0 19.0 1.5 7.9 Excellent Excellent Example 444.9 21.0 1.4 8.5 Excellent Excellent Example 5 27.2 9.5 1.5 10.1 GoodFair Example 6 31.9 16.0 1.3 9.7 Excellent Fair Example 7 33.0 20.0 1.39.5 Excellent Good Example 8 55.1 28.0 1.1 9.5 Excellent ExcellentExample 9 51.9 27.4 1.1 8.3 Excellent Excellent Example 10 26.8 9.0 1.711.8 Fair Fair Example 11 31.7 15.6 1.6 9.5 Good Good Example 12 32.314.7 1.4 10.2 Good Good Example 13 34.3 15.9 1.5 9.5 Good ExcellentExample 14 62.1 21.5 1.1 9.0 Excellent Good Example 15 29.0 8.5 1.6 11.0Fair Fair Example 16 32.0 10.0 1.5 11.5 Fair Good Com. Ex. 1 14.8 2.13.0 10.0 — — Com. Ex. 2 23.4 4.0 2.6 11.1 Poor Poor Com. Ex. 3 30.0 8.01.9 9.7 Fair Poor Com. Ex. 4 31.2 8.5 0.7 13.0 Poor Poor Com. Ex. 5 31.86.0 0.8 12.2 Fair Poor Com. Ex. 6 72.3 38.0 0.4 11.0 Good Poor Com. Ex.7 32.0 26.0 1.0 10.1 Good Poor Com. Ex. 8 44.5 32.0 0.6 11.2 Good PoorCom. Ex. 9 11.0 2.2 3.3 8.5 Poor Poor Com. Ex. 10 75.6 26.0 0.9 12.0Fair Poor Note: ⁽¹⁾With respect to pores open on the cell wall surfaces,a cumulative opening area (%) in a pore opening diameter range of 100 μmor more.

Table 4 indicates that the ceramic honeycomb structures of Examples 1-16(within the present invention) having high strength exhibit highcapability of catalytically removing harmful substance with smallpressure loss. On the other hand, the ceramic honeycomb structures ofComparative Examples 1-10 are poor not only in strength, but also in thecapability of catalytically removing harmful substance.

EFFECT OF THE INVENTION

Because the ceramic honeycomb structure of the present invention ensureshigh flowability of an exhaust gas through pores in the cell walls whileexhibiting high strength, even when it is not provided with plugs, anexhaust-gas-cleaning ceramic honeycomb structure exhibiting highcapability of catalytically removing harmful substance with smallpressure loss can be obtained by carrying a catalyst.

1. A ceramic honeycomb structure comprising large numbers of cellspartitioned by porous cell walls, said cell walls having (a) porosity of50-80%; and when measured by mercury porosimetry, (b) a median porediameter being 25-50 μm; (c) (i) a cumulative pore volume in a porediameter range of 20 μm or less being 25% or less of the total porevolume; (ii) a cumulative pore volume in a pore diameter range of morethan 20 μm and 50 μm or less being 50% or more of the total pore volume;and (iii) a cumulative pore volume in a pore diameter range of more than50 μm being 12% or more of the total pore volume.
 2. The ceramichoneycomb structure according to claim 1, wherein an opening area ratioof pores open on said cell wall surfaces (a total opening area of poresper a unit cell wall surface area) is 30% or more.
 3. The ceramichoneycomb structure according to claim 1, wherein pores open on saidcell wall surfaces have a median opening diameter (expressed by anequivalent circle diameter) of 60 μm or more.
 4. The ceramic honeycombstructure according to claim 1, wherein a cumulative opening area ofpores open on said cell wall surfaces, which have opening diameters(each expressed by an equivalent circle diameter) of 30 μm or less, is20% or less of the total opening area.
 5. The ceramic honeycombstructure according to claim 1, wherein a cumulative opening area ofpores open on said cell wall surfaces, which have opening diameters(each expressed by an equivalent circle diameter) of 100 μm or more, is30-70% of the total opening area.
 6. The ceramic honeycomb structureaccording to claim 1, wherein said cell walls have permeability of10×10⁻¹² m² to 30×10⁻¹² m².
 7. The ceramic honeycomb structure accordingto claim 1, wherein said ceramic honeycomb structure has an A-axiscompression strength of 1.0 MPa or more.
 8. A method for producing aceramic honeycomb structure comprising the steps of blending materialpowder comprising a cordierite-forming material and a pore-formingmaterial to obtain a moldable material, extruding said moldable materialto obtain a honeycomb-shaped green body, and drying and sintering saidgreen body to obtain a ceramic honeycomb structure, (a) saidcordierite-forming material comprising 10-25% by mass of silica having amedian particle diameter of 10-60 μm; (b) said pore-forming materialhaving (i) a median particle diameter of more than 70 μm and 200 μm orless, (ii) in a curve of a cumulative volume (cumulative volume ofparticles up to a particular particle diameter) to a particle diameter,a particle diameter D90 at a cumulative volume corresponding to 90% ofthe total volume being 90-250 μm, and a particle diameter D10 at acumulative volume corresponding to 10% of the total volume being 15-160μm; and (iii) a particle diameter distribution deviation SD being 0.3 orless; and (c) said pore-forming material added being 1-20% by mass persaid cordierite-forming material.